2.0 Analysis 2.1 Introduction The laboratory analysis determined that the down elevator cable failed as a result of gradual deterioration due to corrosion; however, contamination of the physical evidence, which occurred when the aircraft sank, precluded an objective evaluation of the corrosion mechanism. It is believed that moisture collected on the pulley and provided the catalyst for corrosion to take place. Attesting to this are several similar instances of control cable failures. The analysis will focus on the possible reasons why the cable's deterioration progressed to the point of failure without being detected. Two possibilities were considered: first, that the procedures and requirements for inspecting the control cables were inadequate, and second, that the personnel who conducted the inspections did not exercise due diligence in the performance of those inspections. 2.2 Cable Inspection 2.2.1 Procedures and Requirements Since the maintenance history of the aircraft was not complete, the original date of installation of the failed cable and the cable's time in service could not be determined. The cable installed in the accident aircraft met the de Havilland specification and, although some operators use corrosion-resistant stainless steel cables on this and other aircraft types, such cables were not required on this aircraft type. The EMMA 18 maintenance interval for the accident aircraft was in accordance with the parameters of the manufacturer, Transport Canada, and the company. The EMMA number 18 inspection procedure has been proven effective by other aircraft companies in detecting corroded control cables. (See Appendix B.) 2.2.2 Personnel Performance The AME who completed the most recent inspection of the control cables at station 376 was only able to recall lubricating the cables and inspecting non-associated items in the aft fuselage. Following the procedure as laid out in the EMMA number 18 work card, the AME did not remove the aft baggage compartment floor to provide access to both sides of the station 376 pulley group. Nevertheless, when the controls were cycled to the full extent of their travel, it should have been possible for the AME to detect any frayed or discoloured cables. Although he was experienced with the inspection procedure and was aware of control cable failures due to corrosion, it could not be determined with what diligence he performed the inspection. 2.3 Company Maintenance Organization The company Maintenance Control Manual and Inspection Program Approval for the DHC-6 type had been approved by Transport Canada. Airworthiness audits had been conducted, and although some problems had been identified, these had been rectified to the satisfaction of Transport Canada. 2.4 Aircraft Operation 3.0 Conclusions 3.1 Findings The aircraft's weight and centre of gravity position were within limits. The captain and first officer were certified and qualified for the flight. The elevator control cable failed as a result of gradual deterioration due to corrosion. Contamination of the physical evidence by exposure to sea water precluded an objective evaluation of the corrosion mechanism. The uncorroded portion of the cable met the manufacturer's specifications. Autopsy, toxicology, and medical records revealed no evidence to indicate that the first officer's performance was degraded by physiological factors. There was no evidence to indicate that the captain's performance was degraded by physiological factors. It was not possible for the pilots to maintain control of the aircraft after the elevator cable had failed. Neither the captain nor the first officer was wearing the available shoulder harnesses. None of the occupants were wearing the available flotation devices. The control cable corrosion was not detected on the accident aircraft during the last cable maintenance inspection. The pilot selected 30 degrees of flap for take-off; the aircraft flight manual specified 20 degrees of flap for take-off. 3.2 Causes The down elevator control cable failed at station 376 due to corrosion, and, as a result, the pilot lost control of the aircraft. The corrosion was not detected by the maintenance personnel during the aircraft's last cable inspection. 4.0 Safety Action 4.1 Action Taken 4.1.1 Bombardier Inc. On 25 October 1994, Bombardier Regional Aircraft Division (BRAD) issued a service letter to all DHC-6 operators, advising those operators of this occurrence. The service letter advised those operators of existing information regarding cable failures, and noted the following: ... operators receiving Twin Otter maintenance publications from BRAD on microfiche are reminded that prior [to] receiving microfiche revisions, hard (paper) copies of revisions and temporary revisions will continue to be issued and forwarded. It is the operator's responsibility to utilize both microfiche and any hard copy revision when maintaining their aircraft, to ensure that they are using the latest information available. It is also an operator's responsibility when he acquires a new Twin Otter aircraft in his fleet, to fully understand the aircraft's previous maintenance history and the environment in which it was operated. Based upon this understanding, the operator should initiate a detailed inspection of the aircraft prior to entering it into service. BRAD/DHI is currently in the process of reviewing the DHC-6 Maintenance Manual, Inspection Requirements Manual and the EMMA Program with a view of reinforcing existing maintenance instructions for control cable maintenance and inspection. ... Bombardier also issued temporary revisions to the Inspection Requirements Manual. Revision 73 pertains to DHC-6 Twin Otter aircraft operating in areas of high salt content or marine environment (floatplane and land plane). It lists increased corrosion inspection requirements for the elevator and rudder control cables. Most significantly, this revision announces the requirement to replace the control cables every 12 months. Previously, control cables were replaced when their condition warranted. There was no fixed service life. Revision 74 pertains to special inspection requirements for operations with corrosive cargo. In addition, it requires the replacement of the elevator and rudder control cables below the baggage compartment floor if a spill occurs. Revision 75 pertains to DHC-6 Twin Otter aircraft operating with floats installed. It lists increased corrosion inspection requirements for the elevator and rudder control cables. Like revision 73, this revision announces the requirement to replace the control cables every 12 months. Revision 79 adds to and revises the inspection requirements for the elevator and rudder control cables in the Inspection Requirements Manual for the DHC-6 Twin Otter. In addition, it includes instructions for the amendment of the EMMA work cards. The changes address the need for complete access to the inspection areas, detailing specific areas for corrosion inspection, and the requirement to apply corrosion preventive compound to the cables. In addition, NOTES are added that require the replacement of all (including aileron) control cables installed in land planes every 60 months; the NOTES also advise that stainless steel control cables are available. In October 1994, BRAD and DHI issued a DHC-6 SAFETY OF FLIGHT SUPPLEMENT for all aircraft models which states that: de Havilland had recently become aware that some DHC-6 operators are using 30 degree flap for take-off. The only approved flap setting for take-off on DHC-6 aircraft operated on floats is 20 degrees and all performance information provided in the AFM is predicated on using flap 20. Use of 30 degrees flap may degrade aircraft performance in the event of an engine stoppage immediately following liftoff and may adversely affect aircraft handling characteristics during certain centre of gravity and all-up weight conditions. 4.1.2 Transport Canada ... operators carefully inspect, as soon as practicable, the control cables and related components, paying particular attention to areas where these cables pass over the pulleys, including the pulley cluster area under the baggage compartment at fuselage station 376... The controls should be moved to ensure that no segment of a cable is left uninspected. If suspect, the cable should be removed for a more thorough examination. Furthermore, if the aircraft operates in (or has operated in) a corrosive environment, or carries (or has carried) corrosive cargo, the frequency of inspection should be increased according to the manufacturer's recommendations.